1. Field of the Invention
The present invention relates to an actuator using an electromechanical transducer, particularly to an actuator using an electromechanical transducer suitable for finely positioning an optical system of a lens or the like.
The present invention also relates to an apparatus such as an optical system which employs such actuator.
2. Prior Art
There have been proposed actuators using an electromechanical transducer having a piezoelectric element for driving component parts in a camera and other precision equipment. Such actuators are disclosed in U.S. Pat. No. 5,589,723 and Japanese Laid Open Patent Publication No. 10-39359.
Here, an explanation will be given of a basic construction of such an actuator. FIG. 12 is a perspective view showing an actuator by disassembling it into constituent members, FIG. 13 is a perspective view showing a state where the actuator is assembled and FIG. 14 is a sectional view showing the structure of a portion where a drive shaft, a slider block and a pad are frictionally coupled. An actuator 100 is constituted by a frame 111, support blocks 113, 113a and 114, a drive shaft 116, a piezoelectric element 115, a slider block 112, and a pad 118. The drive shaft 116 is supported by the support block 113a and the support block 114 movably in the axial direction. One end of the piezoelectric element 115 is fixedly adhered to the support block 113 and other end thereof is fixedly adhered to one end of the drive shaft 116. The drive shaft 116 is supported such that it can be displaced in the axial direction (arrow mark "a" direction and direction opposed thereto) when a displacement is caused in the thickness direction of the piezoelectric element 115.
The drive shaft 116 penetrates the slider block 112 in the horizontal direction, an opening portion 112a is formed at an upper portion of the slider block 112 which the drive shaft 116 penetrates and an upper half of the drive shaft 116 is exposed. Further, a pad 118 which is brought into contact with the upper half of the drive shaft is fittedly inserted into the opening portion 112a, a projection 118a is installed at an upper portion of the pad 118, the projection 118a of the pad 118 is pushed down by a leaf spring 119 and downward urging force F for bringing the pad 118 in contact with the drive shaft 116 is applied on the pad 118. Incidentally, numeral 121 designates screws for fixing the leaf spring 119 to the slider block 112. The structure of the portion where the drive shaft 116, the slider block 112 and the pad 118 are brought into contact with each other is shown by FIG. 14.
By such a structure, the drive shaft 116, the pad 118 and the slider block 112 are frictionally coupled by pertinent frictional coupling force. Adjustment of the urging force F for determining the frictional coupling force can be controlled by a degree of fastening the screws 121.
Next, an explanation will be given of the operation. First, when a sawtooth wave drive pulse having a gradual rise portion and a steep fall portion as shown by FIG. 15(a) is applied to the piezoelectric element 115, at the gradual rise portion of the drive pulse, the piezoelectric element 115 is gradually displaced to elongate in the thickness direction and the drive shaft 116 coupled to the piezoelectric element 115 is also displaced gradually in the positive direction (arrow mark "a" direction). At this moment, the slider block 112 frictionally coupled to the drive shaft 116 is moved in the positive direction along with the drive shaft 116 by the frictional coupling force and accordingly, a driven member not illustrated which is coupled to the slider block, for example, a frame for holding a correcting lens in the case of a correcting lens drive mechanism can be moved.
At the steep fall portion of the drive pulse, the piezoelectric element 115 is rapidly displaced to contract in the thickness direction and the drive shaft 116 coupled to the piezoelectric element 115 is also displaced rapidly in the negative direction (direction opposed to arrow mark "a"). At this moment, the slider block 112 frictionally coupled to the drive shaft 116 remains unmoved substantially at the position by overcoming the frictional coupling force by inertia force. By continuously applying the drive pulses to the piezoelectric element 115, reciprocating oscillation having different speeds is caused in the drive shaft 116 and the slider block 112 frictionally coupled to the drive shaft 116 can be moved continuously in the positive direction.
Incidentally, "substantially" mentioned here includes a case where the slider block 112 follows the drive shaft 116 while causing a slip on faces where the slider block 112 and the drive shaft 116 are frictionally coupled in either of the positive direction and the direction opposed thereto and the slider block 112 is moved as a whole in the arrow mark "a" direction by a difference in drive time periods.
In moving the slider block 112 in a direction opposed to the previous direction (direction opposed to arrow mark "a"), the movement can be achieved by changing the waveform of the sawtooth wave drive pulse applied on the piezoelectric element 115 and applying a drive pulse comprising a steep rise portion and a gradual fall portion as shown by FIG. 15(b).
According to the actuator using a piezoelectric element mentioned above, it has become apparent by experiments that the elongation displacement characteristic and the contraction displacement characteristic of the piezoelectric element in respect of the same applied voltage differ from each other and accordingly, the drive speeds differ from each other between a case where the sawtooth wave drive pulse having the gradual rise portion and the steep fall portion as shown by, for example, FIG. 15(a) and a case where the drive pulse comprising the steep rise portion and the gradual fall portion as shown by FIG. 15(b) having a waveform where the previous drive pulse is reverted.
Therefore, in order to provide the same drive speed in either of the directions of both in the case of moving the slider block in the positive direction (arrow mark "a" direction) by utilizing the gradual elongation displacement of the piezoelectric element (hereinafter, referred to as elongation displacement drive) and in the case of moving the slider block in the negative direction (direction opposed to arrow mark "a") by utilizing the gradual contraction displacement of the piezoelectric element (hereinafter, referred to as contraction displacement drive), the piezoelectric element may be driven by generating drive pulses respectively having different waveforms in accordance with the elongation displacement drive and the contraction displacement drive.
However, generation of the drive pulses respectively having different waveforms in accordance with the elongation displacement drive and the contraction displacement drive gives rise to inconvenience where not only the structure of a drive pulse generating circuit or a control circuit becomes complicated but also number of parts is increased and the manufacturing cost is increased.
Further, depending on an apparatus to which the above-described actuator is applied, the drive speed of the actuator is changed by a direction of the gravitational force exerted on the apparatus. For example, when the above-described actuator is applied in driving a correcting lens for correcting a shift in holding a camera, load applied on the actuator is varied and the speed for driving the correcting lens is changed in either of a case where the correcting lens is moved in the up and down direction, that is, in the direction of the gravitational force when the optical axis of the photographing lens of the camera is substantially at the horizontal position and in the case where it is moved against the gravitational force.
In this way, depending on an apparatus to which the actuator is applied, there causes inconvenience where load is varied by the direction of the gravitational force exerted on the apparatus and the drive speed is changed or the like.